I've been using a DIY substrate for about 10 years now. Although, as is often pointed out, my set up is not yet a high tech one (I can't afford pressurised CO2 for one thing), I have had very good results with pretty much every plant I have tried, even growing plants described as needing higher light levels in a low light setup without them turning leggy.
There is no Akadama in it though.
It's a mixed substrate of lime free fine gravel, activated carbon (the same sort as sold for us in filters), peat and API Leaf Zone laterite. Oh and re the carbon, the reasons I put it in were at the time there were a few articles around supporting it's use in planted tank substrates for a few reasons:-
1) To increase the biological capacity of the substrate as it has a very very high surface area to volume ratio that can be colonised by benificial bacteria, both filter bacteria and nitrogen fixing bacteria that help plant roots to extract nitrogenous nutrients from the substrate.
2) As commented here re zeolite, to work as a store for excess nutrients, releasing them back into the water column at a later time (kind of links with all the opinions that used to say over time activated carbon leeches stuff back into the water).
I used to recharge this using Dupla Root balls about every 2 years or so, but I neglected this for a while. I've just used some JBL root balls however to help get things moving again.
I don't know how much of this was based on conjecture and opinions, but for me it has always worked well, with healthy plant growth even in both low light no CO2 conditions, and in higher light (about 3.6 watts per gallon) with DIY CO2 and trace dosing (I don't NP dose as as has already been commented upon my tapwater supply is fairly high in these, and also as I have commented before I find I get a lot of nitrogenous nutrients and phosphates from detritus from my animal life and fish food), to the point where I have to prune even my Ammania Senagalensis weekly, my Ludwigia twice weekly and my Bacopa Monnieri every 2 weeks (these been the only stem plants currently in there), all of these pearl nicely, and the ammania and Ludwigia are lovely shades of red and green.
As to Akadama, I'll be seeting up a small (about 5 gallon) planted nano some time around christmas for dwarf shrimp, this would have been great for this but I have the horrible feeling that it would be unsuitable, which is a shame.
Ade